1. Field of the Invention
This invention relates to the preparation of synthetic polyester filaments, yarns, and cords useful as industrial fibers which show improved strength retention when exposed to hydrolytic conditions or when sealed in a rubber and exposed to elevated temperatures. More particularly, the invention relates to a process whereby the free carboxyl end group concentration of synthetic polyesters may be reduced to a low level while producing yarns having good tensile properties.
2. Description of Related Art
Terephthalate polyesters such as those disclosed by U.S. Pat. No. 2,465,319 are produced in significant quantities for a variety of commercial products. Polyethylene terephthalate, one of the better known terephthalate polyesters, is a reaction product of terephthalic acid and ethylene glycol and is produced having a high degree of polymerization forming long linear polymer chains which normally terminate with either a carboxyl end group or a hydroxyl end group. The melt-spinning of high molecular weight polyethylene terephthalate has been successfully employed to produce fibers possessing properties suitable for use in reinforcing rubber articles including pneumatic tires.
In U.S. Pat. No. 3,051,212 to Daniels, it is disclosed that when rubber articles such as tires, belting, and the like are reinforced with fibers or cords comprising synthetic linear polyester, these fibers or cords show superior strength retention during exposure to high operating temperatures when the free carboxyl end group concentration of the polyester which comprises the cords is reduced to less than 15 milliequivalents per kilogram (meq/kg). It has also been observed that there appears to be a correlation between the resistance of linear polyesters of a given carboxyl end group concentration to high running temperatures in rubber and their resistance to hydrolytic conditions which may be encountered within hot rubber structures.
When structures having polyester reinforcing filaments with high carboxyl group concentration are exposed to high operating temperatures under hydrolytic conditions, excessive loss of strength is observed. This strength loss is particularly evident with polyester filaments used as reinforcing cords or fabrics in pneumatic tires which normally are expected to operate in a safe manner at high temperatures caused by high speeds and heavy loads.
Thus, low free carboxyl end group concentrations for polyester are highly desirable because it is believed to reduce degradation of the fiber. U.S. Pat. No. 4,016,142 to Alexander et al. discloses reducing the free carboxyl end groups by modifying the polyester with small amounts of a glycidyl ether which react with the carboxyl end groups present to form free hydroxy end groups. U.S. Pat. No. 4,442,058 to Griffith et al. discloses a method of reducing the number of free carboxyl end groups whereby minor amounts of a low-boiling oxirane compound such as ethylene oxide are added. U.S. Pat. No. 4,543,396 to Arai et al. discloses reaction of a bisoxazoline compound with the free carboxyl end groups.
U.S. application Ser. No. 161,553, filed Feb. 29, 1988, now U.S. Pat. No. 4,839,124, discloses reduction of carboxyl end groups in polyester with certain lactim ethers exemplified by 0,0'-butylene bis-caprolactim.
It is the object of this invention to provide an improved process for controlling and reducing the concentration of free carboxyl end groups in synthetic linear polyesters. A further object of this invention is to provide a process whereby a modified synthetic condensation polyester polymer of increased molecular weight may be obtained. Yet a further object of this invention is to provide a technique for producing high strength fibers, and provide for drawn yarn having high tensile properties.
As used herein synthetic polyesters mean film-forming or fiber-forming condensation products of dicarboxylic acids such as terephthalic acid and glycols of the series HO(CH.sub.2).sub.n OH where n is an integer from 2 to 10. An important example of this class of polyesters is polyethylene terephthalate which may be prepared by a number of well known processes. For use as reinforcing elements in rubber structures, melt-spun filaments of polyethylene terephthalate are drawn to about 3.5 to about 7.0 times their length after melt spinning, twisted into yarns and plied into cords or woven into fabrics. The cords or fabrics are then treated with adhesive and bonded against or into the rubber structures.
Synthetic linear polyester yarns or cords made by known means can be shown to contain a free carboxyl end group concentration of about 20 or more milliequivalents per kilogram (meq/kg). By "free carboxyl end groups" are meant the acid group, ##STR2## the concentration of which may be determined by Pohl's Method as described in Analytical Chemistry, Vol. 26, page 1614, October, 1954. The formation of such free carboxyl end groups, referred to hereinafter as "COOH", results as a natural consequence of the processes normally employed in the production of linear polyester polymers.
Known synthetic linear polyester yarns or cords for use as rubber structure reinforcing elements are preferably made from a polymer having an intrinsic viscosity of from 0.75 to 1.0 g/dl or higher. Whereas synthetic linear polyester filaments used in textile products normally have an intrinsic viscosity of the polymer of from 0.35 to 0.66 g/dl, these filaments are less suitable as rubber reinforcing elements since they are comparatively weak at a given extension level and fatigue to rupture with relatively little flexing. Filaments made from higher intrinsic viscosity synthetic linear polyester are stronger and less susceptible to such flexing fatigue. It has been found, however, that in the production of a polymer of a high intrinsic viscosity by the normal method of extending the polymerization period, an increase in free carboxyl group concentration results. Thus, while a high intrinsic viscosity polymer may be produced to overcome the problem of filament strength and flexing fatigue, at the same time the added carboxyl end group content results in an increase in the amount of strength lost when the filaments are exposed to high temperatures under hydrolytic conditions in rubber.
The carboxyl concentration is determined by dissolving the polyester in a 70/30 O-cresol/chloroform mixture and titrating the solution with 0.05N KOH in methanol. The end point is determined potentiometrically.
By the phrase "intrinsic viscosity", sometimes denominated IV, it is meant to describe the reduced viscosity of the polymer at zero concentration, which may be determined by measuring the flow times of a polymer solution after successive dilutions with fresh solvent, calculating the reduced viscosities, and extrapolating a plot of the reduced viscosities against concentration to zero concentration. The reduced viscosity is obtained from the expression: ##EQU1## where c is the concentration expressed as grams of polymer per 100 milliliter of solvent (g/dl). As used herein, the intrinsic viscosity was measured at 25.degree. C., using a 60/40 mixture of phenol and tetrachloroethane as a solvent in a modified Ostwald viscometer.